Page 215 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch04.qxd 9/29/05 5:09 PM Page 4.43

BUILDING CODES, LOADS, AND FIRE PROTECTION*

BUILDING CODES, LOADS, AND FIRE PROTECTION 4.43

FIGURE 4.14 Flame shields placed on flanges of a span-
drel girder to protect the web against flames.

available furnaces. Also, beams frame into columns and girders in a number of different ways.
In some cases, connections are designed to resist only shear forces. In other cases, full- or partial-
moment connections are provided. In short, given the cost of testing, the complexity of modern struc-
tural systems, and the size of available test facilities, it is unrealistic to assume that test assemblies
can accurately model real construction systems.
In recognition of the practical difficulties associated with testing, ASTM E119 includes two test
conditions, restrained and unrestrained. The restraint that is contemplated in fire testing is restraint
against thermal expansion, not structural restraint in the traditional sense. When an assembly is sup-
ported or surrounded by construction that is capable of resisting expansion, to some degree, thermal
stresses will be induced in the assembly in addition to those due to dead and live loads. Originally, it
was thought that thermal stresses would reduce the fire resistance of many assemblies. However,
extensive research indicated that restraint actually improved the fire resistance of many common
types of floor systems. The two test conditions in E119 recognize the complexity of this issue.
The restrained condition applies when the assembly is supported or surrounded by construction that
is capable of resisting substantial thermal expansion throughout the range of anticipated elevated tem-
peratures. Otherwise, the assembly should be considered free to rotate and expand at the supports and
should be considered unrestrained. Thus a floor system that is simply supported from a structural stand-
point may often be restrained from a fire-resistance standpoint. To provide guidance in the use of res-
trained and unrestrained ratings, ASTM E119 includes examples in an explanatory appendix (Table 4.19)
which indicate that most common types of steel framing systems can be considered to be restrained
from a fire-resistance standpoint. More recently, Gewain and Troup (2001) more fully described and
justified the use of restrained ratings in steel buildings. (“Restrained Fire Resistance Ratings in
Structural Steel Buildings,” Engineering Journal, Second Quarter 2001, AISC, Chicago, Ill.) The
2005 AISC “Specification for Structural Steel Buildings” also provides similar guidance.

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